1. Field of the Invention
The present invention relates to a tone processing method suitable for use in an image forming apparatus such as an electrophotographic copying machine and a printer.
2. Description of the Prior Art
Conventionally, in a digital color electrophotographic copying machine which functions as an image forming apparatus, in order to improve tone reproduction thereof, a halftone color image has been formed by executing a tone processing by combining a multi-value dither matrix control using a threshold matrix with a pulse width modulation for dividing one dot of image into plural values by varying a pulse width for controlling the ON and OFF of a laser beam output used as a light source. As the threshold matrix used for the multi-value dither matrix control, a dot concentration threshold matrix has been used by which excellent tone is obtained when a color original is a picture. With a dot dispersion threshold matrix, tone deteriorates although resolution improves.
According to the conventional tone processing method, however, when a color original where letters are present together with a picture is tone-processed by use of the dot concentration matrix, the letters cannot be excellently reproduced due to inferior resolution thereof although an image of the picture is excellently reproduced. For this reason, for a color original where both a picture and letters are present, a method has been considered in which the original is sectioned into picture areas and letter areas so that a suitable tone processing is executed for each area. By this method, however, a processing circuit of complicated arrangement is required, and since the area sectioning is difficult, there is a possibility that image quality obtained is poor due to errors in the area sectioning.